单例模式

 

创建单例的基本步骤：

1、声明一个单件对象的静态实例，并初始化为nil

2、创建一个类的类工厂方法，生成一个该类的实例，当且仅当这个类的实例为nil时

3、覆盖allocWithZone：方法，确保用户（程序员）在直接分配和初始化对象时，不会产生另一个对象。

4、实现NScopying协议，覆盖release，autorelease，retain，retainCount方法，以此确保单例的状态。

5、在多线程的环境中，注意实用@synchronized关键字，确保静态实例被正确的创建和初始化。

 

#import <Foundation/Foundation.h>

@interface Single : NSObject<NSCopying>

@property (nonatomic,copy) NSString *username;
@property (nonatomic,copy) NSString *email;

+ (id)shareSingle;

@end

#import "Single.h"

//第一步
static Single *single = nil;

@implementation Single

//第二步
+ (id)shareSingle {
    //第五步，加同步锁
    @synchronized(self) {
        if (single == nil) {
            single = [[[self class] alloc] init];
        }
    }
    return single;
}

//第三步
+ (id)allocWithZone:(NSZone *)zone {
    if (single == nil) {
        single = [super allocWithZone:zone];
    }
    return single;
}

//第四步
- (id)copyWithZone:(NSZone *)zone {
    return single;
}

- (id)retain {
    return single;
}

- (NSUInteger)retainCount {
    return 1;
}

- (oneway void)release {
}

- (id)autorelease {
    return single;
}
@end

#import <Foundation/Foundation.h>
#import "Single.h"

int main(int argc, const char * argv[])
{

    @autoreleasepool {
        
        Single *single = [Single shareSingle];
        Single *single2 = [Single shareSingle];
        Single *single3 = [[Single alloc] init];
        Single *single4 = [single copy];
        
        [single release];
        [single release];
        [single release];
        [single release];
        [single release];
        
    }
    return 0;
}

所有地址都是同一个